The production of precision microelectronic devices, such as integrated circuits on silicon chips, requires extremely clean conditions. Significant reductions in device production yield have been traced to submicrometer particle deposition during the fabrication process. Fatal defects can be caused by particles which are a fraction of the minimum feature size of the device. The trend toward decreasing line widths in integrated circuits, etc. places increasing emphasis on the control of contaminant particles substantially smaller than 0.1 micrometer (1,000 angstroms) in the fabrication environment. Although many defects result from airborne contaminants in the clean room, it is also important to control particulate contamination within the process gas distribution system servicing the fabrication facility. High purity gases, such as nitrogen, flow through such systems directly to device processing equipment. Although these gases are typically filtered to high levels of cleanliness at the entrance to the distribution system, their cleanliness can only be assured through an accurate measurement of particulate concentrations within the supply system.
Previous experimental studies have demonstrated particulate concentrations of less than 0.3 per liter for all particles as small as 0.003 micrometer (30 angstroms) in clean room gas delivery systems. Such low particulate concentrations result in a correspondingly low arrival rate of detected particles when sampling with a particle counter. Low rates of particle detection tend to reduce the resolution of the particle detection test. That is, the difference between the particulate detection response and instrument background noise count rate becomes small. Therefore, long sample times (or large sample volumes}are required in order to statistically resolve the particulate count rate. This problem can be minimized by using a particle counter having a low background noise level. Various particle counters for determining the concentration of contaminant particles are known.
However, despite the abundance of prior art directed to counting quantitatively the amount of particle contamination in a gas, it is also important to not only determine the concentration of fine contaminant particles within the supply system, but to also determine their sizes so as to give a qualitative detection. Information regarding the size distribution of contaminant particles is as important in assessing their impact for the device fabrication process. Particle size determination should be performed to as small a particle as possible in order to meet the future as well as the present needs of the electronics industry. Therefore, a useful particle measuring device should determine the size distribution of fine, i.e., less than 0.1 micrometer, contaminant particles, with a low rate of spurious counts generated by instrument background noise.
Previous attempts to obtain continuous low noise sizing of fine contaminant particles have included laser particle spectrometers. These instruments determine the equivalent optical diameters of contaminant particles through a process of light scattering from individual particles. The intensity of scattered light is related directly to optical particle diameter through a separate calibration using particles of known diameter and refractive index. Such instruments typically classify particles into discrete size ranges (i.e., 0.1 to 0.2 micrometer, 0.2 to 0.3 micrometer, etc.). Continuous sizing of particles is not normally performed. Modern laser particle spectrometers typically function with low background noise for particles larger than 0.1 micrometer, but are noise limited in lower size detection capability because of light scattering from the subrange particles and gas molecules. Accurate particle sizing also depends upon the usually unknown refractive index and morphology of contaminant particles. In addition, the calibration of size versus scattered light intensity is subject to multivalued response due to resonances in the scattering function for certain ranges of particle diameter. This reduces the confidence in overall particle size determination provided by the instrument.
Previous attempts to obtain low noise particle detection below 0.1 micrometer have included condensation nucleus counters. These instruments use continuous conductive cooling, continuous cooling through dilution or cooling through expansion to create a supersaturated aerosol mixture. Various substances have been used as a saturating medium, including water, alcohol, such as butanol, and perfluorinated organic compounds, such as perfluorodimethyldecalin. The fine particles act as nucleation sites for vapor condensation and subsequent droplet growth. Droplets grow to sufficient size to permit detection by conventional light scattering or light absorption techniques with negligible accompanying noise.
Such a condensation nucleus counter has been described in U.S. Pat. No. 4,790,650 wherein a device admits a gaseous flow into a saturator zone and then takes a portion of the flow through a chilled region to condense a working fluid on entrained particles to enlarge the diameter of the particle to facilitate counting by downstream means, such as an optical detection device. The text of this patent is incorporated by reference herein in its entirety.
Additional descriptions of condensation nucleus counters are found in the dissertation by M. R. Stolzenburg, particularly Chapter 5, titled "An Ultrafine Aerosol Condensation Nucleus Counter", and an article "A Condensation Nucleus Counter Design for Ultrafine Particle Detection Above 3 nm Diameter" by P. B. Keady, V. L. Denler. G. J. Sem, M. R. Stolzenburg and P. H. McMurray.
U.S. Pat. No. 4,293,217 discloses a continuous flow condensation nucleus counter and process for detecting small particle contaminants in gas streams.
U.S. Pat. No. 4,128,335 discloses a condensation nucleus counter with automatic ranging to determine particle sizing.
Additional patents include U.S. Pat. No. 3,806,248 and U.S. Pat. No. 3,632,210.
The theory for the operation of one type of condensation nucleus counter is set forth in an article by M. R. Stolzenburg and P. H. McMurray, entitled "Counting Efficiency of an Ultrafine Aerosol Condensation Nucleus Counter: Theory and Experiment".
Condensation nucleus counters are capable of detecting individual particles as small as 0.003 micrometer (30 angstroms) with negligible noise. However, the final droplet size is relatively uniform and independent of the original particle size. Therefore, information regarding the original contaminant particle size is lost in the condensation process. Therefore, the condensation nucleus counter when operated according to previous methods does not provide information on particle size distribution.
In order to use the condensation nucleus counter for measuring particle size distributions according to previous methods, upstream particle size selectors were required. These devices removed all particles from the contaminant gas stream, except those near a selected size or except those larger than the selected size. Examples of particle size selectors include electrostatic classifiers and diffusion batteries. The condensation nucleus counter in combination with the size selector can then be used to size contaminant particles and to measure their relative concentrations in the gas. However, the size selectors have been found to produce significant numbers of spurious particles through processes such as shedding and electrode sputtering. Therefore, these size selectors are of limited value in measuring the contamination levels of ultra clean systems requiring a low background noise.
Other techniques for measuring fine particle size distributions include particle capture on filters or impaction devices. Particle size distributions are then obtained using microscopy, gravimetric techniques or other methods. These techniques are tedious, expensive, sensitive to subjective interpretation and require batch sampling. In addition, the sampling times required to obtain measurable quantities of particulate matter from ultra clean gas systems is long when using these techniques.
Accordingly, there exists a need in the area of ultra clean gas handling and supply for a rapid, continuous, sensitive technique for measuring not only quantitative but qualitative parameters, specifically size, of submicrometer sized particles. The present invention as set forth below overcomes the disadvantages set forth above of the prior art and achieves the goal of rapid, continuous, sensitive determination of size in ultra clean gas systems.